1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device such as a field-effect transistor (FET) having a heterostructure of gallium nitride-based semiconductor which is generally represented as (InXAl1-X)YGa1-YN (where 0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61).
2. Description of the Related Art
A gallium nitride-based semiconductor such as GaN, AlGaN, InGaN, InAlGaN or the like has a high dielectric breakdown field, high thermal conductivity and a high electron saturation velocity, and thus is promising as a material for a high-frequency power device. Particularly in a semiconductor device having an AlGaN/GaN heterojunction structure, electrons accumulate with high density in the close vicinity of a heterojunction interface between AlGaN and GaN, and a so-called two-dimensional electron gas is formed. This two-dimensional electron gas exists in a spatially separated state from donor impurities added to AlGaN, and thus shows high electron mobility. A field-effect transistor having such a heterostructure is produced so that a source resistance can be reduced. Moreover, a distance d from a gate electrode to the two-dimensional electron gas is typically as short as tens of nanometers, and thus, even if a gate length Lg is as short as about 100 nm, the ratio of the gate length Lg to the distance d (i.e., aspect ratio) Lg/d, can be increased from 5 to about 10. Accordingly, semiconductor devices having an AlGaN/GaN heterostructure have a superior feature in that a field-effect transistor which has an insignificant short-channel effect and satisfactory saturation property can be readily produced. Moreover, a two-dimensional electron of the AlGaN/GaN-based heterostructure has an electron velocity in a high field region of about 1xc3x97105 V/cm, which is twice or more than the electron velocity in AlGaAs/InGaAs-based heterostructure currently prevalent as a high-frequency transistor, and thus, is expected to be applied to high-frequency power devices.
FIG. 8 shows an exemplary cross-sectional view of a conventional FET 800 having an AlGaN/GaN-based heterostructure. The AlGaN/GaN-based heterostructure of the FET 800 is typically formed on a substrate 801 composed of a [0001] facet (c facet), through a crystal growth process using a metal-organic chemical vapor deposition method or a molecular beam epitaxy method. Typically, a sapphire substrate or SiC substrate is used as the substrate 801. In the FET 800, a buffer layer 802 including GaN and an electron supply layer 805 including AlGaN are sequentially provided on the sapphire or SiC substrate 801. On the electron supply layer 805, a source electrode 806, a gate electrode 807, and a drain electrode 808 are provided separately from one another. In the case of forming the buffer layer 802 including GaN on the sapphire or SiC substrate 801, it is necessary to thickly form the buffer layer 802 in order to account for a great lattice constant difference between the substrate 801 and the buffer layer 802. This is because the strain due to a lattice mismatch between the buffer layer 802 and the substrate 801 is sufficiently relaxed by forming the buffer layer 802 so as to have a relatively large thickness. By forming the electron supply layer 805 containing AlGaN to which n-type impurities, such as Si or the like, are added so as to have a thickness on the order of tens of nanometers on the thick buffer layer 802, a two-dimensional electron gas (i.e., electron channel) is formed in the buffer layer 802 which has a great electron affinity in the heterointerface between the buffer layer 802 and the electron supply layer 805 (i.e., between AlGaN and GaN) due to the effects of selective doping. The crystal facet of a heterostructure formed by an MOCVD (Metal-Organic Chemical Vapor Deposition) method, is typically composed of a facet of Ga, which is a group III element. This two-dimensional electron gas is susceptible to the effects of piezo-polarization in a c axis direction due to tensile stress imposed on AlGaN, in addition to a difference in spontaneous polarization between AlGaN (included in the electron supply layer 805) and GaN (included in the buffer layer 802). Thus, electrons accumulate at a density which is higher than a value which would be expected from the density of the n-type impurities added to the electron supply layer 805. When the Al composition of AlGaN of the electron supply layer 805 is 0.2 to 0.3 with respect to AlGaN, electron density of the channel layer formed in the buffer layer 802 is about 1xc3x971013/cm2, which is about 3 times the density of a GaAs-based device. Since the two-dimensional electron gas at such a high density is accumulated, the semiconductor device 800 used as a GaN-based heterostructure field-effect transistor (FET) is considered as a highly promising power device.
However, such a conventional FET 800 has several disadvantages. The first of which is that due to the immaturity of crystal growth techniques, a crystal with satisfactory quality cannot be obtained.
One of the problems related to the crystal growth is associated with the fact that the undoped GaN included in the buffer layer 802 typically is an n-type and the carrier density may be as high as about 1016/cm3 or more. This is presumably because constituent nitrogen (N) atoms are released during crystal growth, and thus, vacancies are liable to be formed. When there are such residual carriers, the leakage current component via the buffer layer 802 of the device becomes greater. In particular, when operating the device at a high temperature, deteriorations in the element properties, such as aggravation of pinch-off characteristics, may occur. As for an isolation problem, when forming a plurality of GaN-based heterostructure FETs on the same substrate, the FETs interfere with each other, hindering normal operation. When the gate electrode 807 is further provided above this buffer layer 802, a problem such as an increase of a gate leakage current, or the like, may arise.
The second disadvantage of a conventional FET 800 is ascribed to the effects of polarization as described above. In a conventional FET having an AlGaAs/InGaAs-based heterostructure, a channel layer is composed of InGaAs, and an electron (carrier) supply layer is composed of AlGaAs and is doped with Si. In general, when such a FET is applied as a power device, an AlGaAs/InGaAs/AlGaAs structure, in which an InGaAs (channel) layer is sandwiched by two n-type AlGaAs layers, is employed. In this structure, the electron density of the channel layer is about 2 times the electron density of a channel layer in a non-sandwich type structure. FIG. 9 schematically shows a distribution of the potential energy of conduction band along the depth direction of such a semiconductor device. As shown in FIG. 9, electrons are supplied from the AlGaAs layers to the InGaAs layer whose potential is lower than those of the AlGaAs layers. Such a structure which has two Si-doped AlGaAs layers is called a double-doped structure or a double-doped, double-heterostructure.
FIG. 10 shows a structure of an n-type AlGaN/GaN/n-type AlGaN device 1000, which is a GaN-based device having a double-doped structure. FIG. 11 shows a distribution of the potential energy along the depth direction in the semiconductor device 1000.
The conventional FET 1000 shown in FIG. 10 sequentially includes the following layers on a sapphire or SiC substrate 1001: a first channel layer 1002, including GaN; a first electron supply layer 1013, including AlGaN; a second channel layer 1004, including GaN; and a second electron supply layer 1005, including AlGaN. On the second electron supply layer 1005, a source electrode 1006, a gate electrode 1007 and a drain electrode 1008 are provided separately from one another. In the GaN-based double-doped structure shown in FIG. 10, doping is performed only on the second electron supply layer 1005 because a large polarization influence is caused on electrons which are supplied from the first and second electron supply layers 1013 and 1005 to the first and second channel layers 1002 and 1004, respectively.
As seen from the graph of FIG. 11, in a GaN-based semiconductor device, the potential in a heterointerface between the second channel layer 1004 and the first electron supply layer 1013 is significantly increased due to piezo-polarization or spontaneous polarization. As a result, electrons accumulate in two separate regions. That is, a first (lower) electron channel is formed by electrons which have accumulated in the first channel layer 1002 in the close vicinity of a heterointerface with the first electron supply layer 1013, and a second (upper) electron channel is formed by electrons which have accumulated in the second channel layer 1004 in the close vicinity of a heterointerface with the second electron supply layer 1005. Currents flow through these electron channels. The distance between the first and second electron channels is about several tens of nanometers. With such a great distance, the mutual conductance of the conventional FET 1000 is small as compared with an AlGaAs/InGaAs/AlGaAs structure in which electrons accumulate so as to form a single electron channel. As a result, the gain of the conventional FET 1000 is decreased, which is undesirable in view of high frequency operation.
According to one aspect of the present invention, a semiconductor device includes: a substrate; a buffer layer including GaN formed on the substrate, wherein surfaces of the buffer layer are c facets of Ga atoms; a separating layer including (InXAl1-X)YGa1-YN (where 0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61) formed on the buffer layer, wherein surfaces of the separating layer are c facets of In, Al, or Ga atoms; a channel layer including GaN, InGaN, or a combination of GaN and InGaN formed on the separating layer, wherein surfaces of the channel layer are c facets of Ga or In atoms; and an electron supply layer including AlGaN formed on the channel layer, wherein surfaces of the electron supply layer are c facets of Al or Ga atoms, wherein the AlN composition ratio in the separating layer is smaller than the AlN composition ratio in the electron supply layer.
In one embodiment of the present invention, in the separating layer including (InXAl1-X)YGa1-YN, X=0.
In another embodiment of the present invention, the AlN composition ratio in the separating layer is equal to or smaller than about 0.1.
Instill another embodiment of the present invention, the AlN composition ratio in the separating layer gradually increases from an interface with the buffer layer to an interface with the electron supply layer.
According to another aspect of the present invention, a semiconductor device includes: a substrate; a buffer layer including GaN formed on the substrate, wherein surfaces of the buffer layer are c facets of Ga atoms; a first electron supply layer including (InXAl1-X)YGa1-YN (where 0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61) formed on the buffer layer, wherein surfaces of the first electron supply layer are c facets of In, Al, or Ga atoms; a channel layer including GaN, InGaN, or a combination of GaN and InGaN formed on the first electron supply layer, wherein surfaces of the channel layer are c facets of Ga or In atoms; and a second electron supply layer including AlGaN formed on the channel layer, wherein surfaces of the electron supply layer are c facets of Al or Ga atoms, wherein the AlN composition ratio, the InN composition ratio, and the GaN composition ratio in the first electron supply layer are set such that electrons accumulate in a vicinity of a heterointerface between the first electron supply layer and the channel layer due to a variation in polarization.
According to still another aspect of the present invention, a semiconductor device includes: a substrate; a buffer layer including GaN formed on the substrate, wherein surfaces of the buffer layer are c facets of N atoms; a first electron supply layer including AlGaN formed on the buffer layer, wherein surfaces of the first electron supply layer are c facets of N atoms; a channel layer including GaN, InGaN, or a combination of GaN and InGaN formed on the first electron supply layer, wherein surfaces of the channel layer are c facets of N atoms; and a second electron supply layer including (InXAl1-X)YGa1-YN (where 0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61) formed on the channel layer, wherein surfaces of the electron supply layer are c facets of N atoms, wherein the AlN composition ratio, the InN composition ratio, and the GaN composition ratio in the first electron supply layer are set such that electrons accumulate in a vicinity of a heterointerface between the second electron supply layer and the channel layer due to a variation in polarization.
Thus, the invention described herein makes possible the advantages of: (1) providing a semiconductor device in which a leakage current between a source and a drain, a gate leakage current, and a leakage current between devices caused by residual carriers caused in a GaN layer (channel layer) are significantly reduced; and (2) providing a semiconductor device having a double-doped, GaN-based heterostructure in which electrons can accumulate so as to form a single electron channel and which provides a superior mutual conductance and a high current driving performance.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.